I've used the Scrap Your Boilerplate and Uniplate libraries in the Haskell programming language, and I would find that form of generic programming over discriminated unions to be really useful. Is there an equivalent library in the f# programming language?
+7
A:
Not that I know of; without support built-in to the language/compiler, I expect the only alternative is a reflection-based version. (I don't know how Uniplate is implemented - do you?)
Here's the code for a reflection-based version based on the example from the original presentation. I have not thought deeply about its limitations, but this was much simpler to write than I would have guessed.
type Company = C of Dept list
and Dept = D of Name * Manager * SubUnit list
and SubUnit = | PU of Employee | DU of Dept
and Employee = E of Person * Salary
and Person = P of Name * Address
and Salary = S of float
and Manager = Employee
and Name = string
and Address = string
let data = C [D("Research",E(P("Fred","123 Rose"),S 10.0),
[PU(E(P("Bill","15 Oak"),S 5.0))])]
printfn "%A" data
open Microsoft.FSharp.Reflection
let everywhere<'a,'b>(f:'a->'a, src:'b) = // '
let ft = typeof<'a> // '
let rec traverse (o:obj) =
let ot = o.GetType()
if ft = ot then
f (o :?> 'a) |> box // '
elif FSharpType.IsUnion(ot) then
let info,vals = FSharpValue.GetUnionFields(o, ot)
FSharpValue.MakeUnion(info, vals |> Array.map traverse)
else
o
traverse src :?> 'b // '
let incS (S x) = S(x+1.0)
let newData = everywhere(incS, data)
printfn "%A" newData
The everywhere
function traverses the entire structure of an arbitrary DU and applies the function f
to each node that is the type that f
works on, leaving all other nodes as-is.
Brian
2010-08-29 21:52:34
This is a good suggestion, I'll have to think about the performance implications of such an approach. It probably doesn't matter for my particular usecase anyway.
Richard Warburton
2010-08-29 23:16:11
re: your question about how Uniplate is implemented the source is available at http://community.haskell.org/~ndm/darcs/uniplate/ .
Richard Warburton
2010-08-29 23:17:27
I've accepted your answer since I think its a good approach - but I make a change in order to get it to work correctly:I replaced 'ft = ot' with 'ot.IsSubclassOf(ft)' - otherwise it fails to match when the argument type of f, ie 'a is more generic than the specific argument being passed.
Richard Warburton
2010-08-30 18:39:36